For those that want to know, Mr Bunny and the rest of the group were fighting Vinnara from the World's Largest Dungeon, Room N102.
She is Ghost Fighter 9/Rog 1/Blackguard 10 and CR 24, also in the room were 4 Dread Wraiths which had been increase by HD. The book lists it as an EL 23 fight. The party was made up of 6 characters of around level 13. While the DMG may say that party's shouldn't be in able to overcome this sort of encounter they did in the end, they were able to prepare and had 2 clerics who were able to cast heal, which providing they hit can do massive amounts of damage even when you save for half. Don't get me wrong this was no easy fight, it used up all the resources they had and in the end Mr Bunny was the only one left alive.
As far as exp goes the chart in the dmg doesn't give you any amount saying that the fight should be too hard and that it's left at the DM's discression as to how much exp to hand out, there is nothing in the DMG on how to work it out using formulas or anything like that. However we are wondering if anyone out there has expanded on the table in the DMG to include how much exp a level 13 character should get for defeating a EL 23 fight. The Epic level handbooks lists exp for character level 21+ so that's no use here.
Saying he should get enough exp to make him one point on 15th level is way too much, and saying he should get none just cos the book says he shouldn't be able to overcome it is stupid.
Has anyone out there sat down and worked out any how the DMG works out the exp for each level vs CR and been able to expand on that?